Abstract

The Overflow Parameterization (OFP) for the ocean component of the Community Climate System Model (CCSM) is presented. The ocean component is based on the Parallel Ocean Program Version 2, or POP2, of the Los Alamos National Laboratory (Smith et al., 2010). The OFP has two parts: the method of evaluating overflow properties based on ocean model state, and the modifications to the baroclinic and barotropic solutions to reflect the influence of the overflows. Evaluating overflow properties involves computing source water transport based on regional averages over an interior and source domain. The source overflow is implemented as a sidewall boundary condition on raised bottom topography, which replaces explicitly resolved overflow with the parameterized overflow. The parameterized overflow is assumed to flow through the ridge (implicitly) and to descend unmodified to an entrainment sidewall, where entrainment ambient water is mixed with the source water, producing the final product water. The product density is computed for the mix of source and entrainment waters, and the product injection position is found where neutral buoyancy occurs along a pre-specified product path. In POP2 the baroclinic and barotropic modes are split. Our approach is to solve the baroclinic equations without change but to modify the barotropic equation to include the effect of the parameterized overflows. We assume that the overflow sidewall velocities are total velocities at each time step, and this non-zero sidewall boundary condition is used to modify the barotropic continuity equation, as well as the vertical planar column velocities above each sidewall at source, entrainment and product locations so that local mass conservation is enforced. Overflow tracer advection is done through the source, entrainment and product sidewalls in a conservative manner. The POP2 implementation allows the user to select overflow source, entrainment and product locations and orientations. It allows for more than one product location so that the product depth can adjust to varying forcing. Various parameters can be adjusted, ideally within observational constraints, to yield overflow transports within observational uncertainties. The implementation method should be applicable to z-coordinate ocean models with the baroclinic/barotropic split as in POP2. For a thorough discussion of the climate impacts of OFP, see Danabasoglu et al. (2010).